Mobile radiography system and grid alignment

ABSTRACT

Disclosed is a mobile radiographic unit with improved x-ray scatter control. Improved x-ray scatter control is provided through the alignment of the system with the focal line of an anti-scatter grid. In a preferred embodiment, the system comprises an x-ray source assembly, a tube housing mounting, a measuring system, a motion control system and a processor in communication with the measuring system and the motion control system.

CROSS REFERENCE TO RELATED APPLICATIONS

The present disclosure is a continuation of U.S. patent application Ser.No. 11/349,424, filed 2-7-2006, now U.S. Pat. No. 7,581,884, issued Sep.1, 2009.

FIELD OF THE INVENTION

The present disclosure relates to radiography and particularly to mobileradiography.

BACKGROUND

In the hospital setting, mobile radiographic exams are performed onpatients that are incapable of being moved, or are difficult to move. Intertiary care medical centers, mobile radiographic exams represent asignificant percentage of the radiographic exams performed. X-rayspassing through an object, such as a human body, experience some degreeof scatter associated with interactions of the x-rays with atoms orelectrons. The primary x-rays transmitted through an object travel on astraight line path from the x-ray source (also referred to herein as thex-ray focal spot) to the image receptor and carry object densityinformation. Scattered x-rays form a diffuse image that degrades primaryx-ray image contrast. In thick patients, scattered x-ray intensityexceeds the intensity of primary x-rays. Scattering phenomena is wellknown and routinely compensated for in general radiography, fluoroscopyand mammography through the use of anti-scatter grids.

An anti-scatter grid includes a laminate of lead foil stripsinterspersed with strips of radiolucent material (FIG. 1). The grid ispositioned between the object to be imaged and the image receptor andoriented such that the image forming primary x-rays are incident onlywith the edges of the lead foil strips. Thus, the majority of primaryx-rays pass through the radiolucent spacer strips. In contrast,scattered x-rays are emitted in all directions due to interaction withthe object and as such, scattered x-rays are incident on a larger areaof the lead strips and only a small percentage of scattered x-rays reachthe image receptor, as compared to primary x-rays. The degree of scattercontrol for a given grid depends upon the grid ratio, which is definedas the ratio of the radiopaque strip thickness in the direction of thex-ray path to the width of the radiolucent spacer material as measuredorthogonal to the x-ray beam path. Thus, the higher the grid ratio, thegreater the scatter control. A high grid ratio, while more effective inreducing scattered x-rays, is also more difficult to align relative tothe x-ray focal spot. In order to compensate for x-ray beam divergencein a focused grid, the radiopaque strips are tilted to a greater extentwith increasing distance from the center of the grid. The planes of thegrid vanes all converge along a line known as the focal line. Thedistance from the focal line to the surface of the grid is referred toas the focal length of the grid. The focal line coincides with thestraight line path to the focal spot (illustrated in FIG. 2). Thus, whenthe focal spot is coincident with the focal line of the grid, theprimary x-rays have minimal interaction with the radiopaque lead stripsand maximal primary transmission is obtained. Misalignment of the focalline of the anti-scatter grid with the focal spot diminishes primaryx-ray transmission while scattered x-ray transmission remains unchanged.Thus, optimal primary x-ray transmission requires alignment (positionaland orientation) of the focal spot with the focal line of theanti-scatter grid.

In general radiography, fluoroscopy and mammography, the image receptorand x-ray tube housing (comprising the x-ray source) are rigidly mountedand in a fixed position relative to one another, thereby making focalspot and grid alignment a simple process. In mobile radiography, animage receptor is placed under a bedridden patient and the x-ray source,mounted at the end of a jointed arm, is positioned above the patient.Since the relative separation of the focal spot and the image receptoris variable, determining the proper position and orientation of ananti-scatter grid between a patient and the image receptor becomes adifficult alignment problem. If a grid is not used, only a smallfraction of the possible contrast is obtained in the x-ray image. As aresult, scatter to primary x-ray ratios of 10:1 or more are common inchest and abdominal bedside radiography resulting in less than 10% ofthe possible image contrast being obtained in mobile radiographic films(1,2). Contrast limitations are exacerbated if digital storage phosphorimage receptors are utilized in place of the more conventionalscreen-film systems (3).

When grids are utilized in conjunction with mobile radiography, the gridis typically not aligned. Misalignment problems are diminished byutilizing a grid having a low ratio of 8:1 or less. Although x-ray imagecontrast is improved with the use of a low ratio grid as compared tocurrent clinical practice, the contrast remains significantly lower thanotherwise could be obtained with a properly aligned, high ratio gridhaving a grid ratio of 10:1 or greater.

Thus while mobile radiography is in many ways more convenient than fixedinstallation radiography, its clinical utility is diminished due to theinferior image quality caused by scattered radiation which is a greaterproblem in mobile radiography due to the difficulty in producing theproper alignment of the focal spot with the anti-scatter grids. A meansto produce proper alignment that is easy for the operator to use wouldsignificantly improve mobile radiographic image contrast and imagequality, and thus increase the clinical utility of mobile radiography.

The prior art has contains a number of mobile radiography systems;however, these system have been limited in their utility in clinicalacceptability owing to the considerable additional effort required onthe part of a radiography technologist to align the x-ray source or thecost and complexity of the systems described. Furthermore, these priorart system are too costly/complex to manufacture to be placed in routineoperation. Therefore, there exists a need for a mobile radiographysystem having a simple, cost effective method to place the focal spotand the central x-ray beam in correct alignment (position andorientation) with regard to the anti-scatter grid. The presentdisclosure provides such a mobile radiography system and method for usetherewith.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an anti-scatter grid common in the field;

FIG. 2 is a schematic view of a focused anti-scatter grid common in thefield;

FIG. 3 is a side view of one embodiment of a mobile radiography systemaccording to the present disclosure;

FIG. 4 is an illustration of the optimal and acceptable state ofalignment for the mobile radiographic system of the present disclosure;

FIG. 5 is a perspective detail view of one embodiment of a target armand fiducial markers according to the present disclosure;

FIGS. 6A and 6B are perspective views of one embodiment of a grid tunnelof the present disclosure;

FIG. 7 is a flowchart illustrating the steps involved in the calibrationprocedure;

FIG. 8 is a flowchart illustrating the steps involved in the alignmentprocedure;

FIG. 9 is a flowchart illustrating the steps involved in measuring theposition of the x-ray source assembly;

FIG. 10 is a flowchart illustrating the steps involved in acquiring animage of the target array;

FIG. 11 is a flowchart illustrating the steps involved in the procedureof localizing individual fiducial markers on the target array;

FIG. 12 is a flowchart illustrating the steps involved in thecalculating the position of the x-ray source assembly; and

FIG. 13 illustrates one embodiment of the x-ray source assembly showingthe different degrees of freedom in each component.

DETAILED DESCRIPTION

The present disclosure provides a device and method to increase x-rayscatter control of mobile radiography equipment through optimalalignment of a focal spot with the focal line of an anti-scatter grid. Amobile radiography device and method according to the present disclosureaffords a rapid and accurate alignment between a mobile radiographicdevice focal spot and the focal line of the anti-scatter grid. In oneembodiment, the present disclosure describes a system generallycomprising an x-ray source assembly, a measuring system, a motioncontrol system and a processor to receive data from and transmitinformation to at least one of the measuring system and the motioncontrol system. The measuring system utilizes a series of positionalencoders for at least one degree of freedom in the mobile radiographicsystem, and a detecting element attached to a mobile radiographic systemto provide position and orientation information of the anti-scatter gridrelative to the radiographic system. The detecting element may be asingle component, or a plurality of components. In one embodiment, thedetecting element comprises a target array or other external object in afixed position and orientation relative to the anti-scatter grid and adetector, such as a camera to provide such position and orientationinformation. A positional encoder is associated with at least one degreeof freedom of the x-ray source assembly; the position and orientation ofthe x-ray source assembly can be determined from values of theseencoders. The detecting element and positional encoders transmits thisinformation to the processor which determines the position of theanti-scatter grid relative to a fixed point on the mobile radiographysystem. The processor determines the alignment of the focal spot andcentral x-ray beam relative to the anti-scatter grid for production ofan optimal image. A motion control system allows alignment of the mobileradiographic system to position the x-ray focal spot to a state ofalignment relative to the focal line of an anti-scatter grid. The motioncontrol system utilizes a directing element in communication with saidprocessor to aid in this process. The directing element guides theoperator in adjusting the degrees of freedom of the mobile radiographysystem to position the x-ray focal spot to a state of alignment relativeto the focal line of an anti-scatter grid. In one embodiment, thedirecting element is visual in nature, such as a visual display; in analternate embodiment, the directing element is tactile in nature, suchas a tactile display; in yet another embodiment, the directing elementis auditory in nature, such as an auditory response. Regardless of theform of the directing element, the directing element informs theoperator when at least one degree of freedom of the mobile radiographysystem has a desired value and provides information necessary to movethe system into a state of alignment. The directing element may informthe operator of the current position of at least one degree of freedomof the mobile radiography system and the desired position of at leastone degree of freedom of the mobile radiography system, or it may informthe operator of the direction in which at least one degree of freedommust be changed to move such degree of freedom to its desired value. Inone embodiment, the directing element displays this information in realtime. The directing element may also provide the operator withinformation regarding the status of the mobile radiographic system.

It is to be appreciated that the position and orientation of any rigidbody can be described by six parameters. In some parameterizations,three parameters describe the position of a fiducial point in the rigidbody, and the remaining three parameters describe the rotation of thebody about this fiducial point. The position may be described in termsof Cartesian coordinates, spherical coordinates, cylindricalcoordinates, or some other coordinate system. The rotation may bedescribed in terms of an unit eigenvector of rotation and a rotationangle, by roll, pitch, and yaw angles, or some other coordinate system.Such parameterizations are made in relation to a specified frame ofreference, and techniques to convert from one frame of reference toanother are well known to those skilled in the art. In thisspecification, the term “Cartesian coordinates” will be used to describeparameterizations useful in converting from one frame of reference toanother. Alternately, the position and orientation may be described bysix parameters that determine position and orientation together ratherthan independently. An example of this is the six parameters H, R, Θ, Ω,Ψ, and Φ shown in FIG. 13. Such parameterizations are useful fordescribing the position and orientation of an object on a jointed arm,such as that illustrated in FIG. 13. In this specification, the term“degree of freedom” shall mean the motion of a single rotational orsliding joint in the arm holding the x-ray source assembly, or for themotion of a combination of joints constrained such that any point on thex-ray source assembly follows a path in space. In this specification,the term “value of the degree of freedom” shall mean a parameterdescribing the state of a single degree of freedom.

In the embodiment illustrated, the motion control system is manuallydriven by the technician/operator. In one embodiment, a high ratioanti-scatter grid is employed; however, any anti-scatter grid known inthe art may be used. For the purpose of this specification, a high ratioanti-scatter grid is defined as a grid having a grid ratio of 10:1 orgreater. Through the device and method of the present disclosure, theprocess of positioning the components of a mobile radiographic system toa state of alignment can be achieved with minimal operator involvement.Furthermore, the system can be produced at low cost with minimalcomplexity.

Referring now to FIG. 3, a particular embodiment of the mobileradiographic system 10 is described and includes a movable base 12, anoperator's console 14, an x-ray source assembly and a tube housingmounting. The x-ray source assembly preferably has at least one degreeof freedom and comprises an x-ray tube housing 22 containing an x-raysource, the tube housing 22 having an x-ray emission aperture (notshown), and a collimator 24 attached to the tube housing 22. The tubehousing mounting has at least one degree of degree of freedom to allowthe x-ray source assembly to be positioned at a desired position andorientation (FIG. 13). In one embodiment, the tube house mountingcomprises an adjustable, vertical column 16, an adjustable, horizontalarm 20 mounted to the column 16 and an adjustable gimbal 23 for couplingthe x-ray tube housing 22 to the arm 20. The foregoing is illustrativeof an x-ray mobile radiographic system known in the art; variations tothe foregoing known to those of ordinary skill in the art are meant tobe included in the scope of the present disclosure.

The mobile system 10 further comprises a processor and a measuringsystem. In this embodiment, the detecting element of the measuringsystem comprises a detector 26 (described in the presented embodiment asan optical detector, or camera) and a target array 28 in a fixedposition and orientation relative to the anti-scatter grid; as discussedabove the measuring system also comprises at least one position encoder.Each of the detector and positional encoder are in communication withthe processor. Furthermore, a light source 25 may also be incorporatedon the mobile radiographic system 10 as described herein.

In one embodiment, the target array 28 is placed in a fixed position andorientation relative to the anti-scatter grid 32 through a target arm 36removably coupled to the grid tunnel 30. The grid tunnel 30 contains theanti-scatter grid 32 and the image receptor 34. In one embodiment, thegrid tunnel 30 comprises a channel to receive the target arm 36(referred to as a channel 40) (FIGS. 6A and B). The target array 28comprises a plurality of fiducial markers 50. In one embodiment, thedetector 26 is attached to the mobile radiographic system 10. Theplacement of the detector 26 may be varied as desired.

The detector 26 is positioned to acquire information, such as but notlimited to an image, regarding the position and orientation of thetarget array 28 and its fiducial markers 50. In one embodiment, theinformation is an image; throughout the remainder of the specification,the information will be referred to as an image. The image istransmitted to the processor and the processor determines the positionand orientation information of the grid tunnel 30 and therefore theanti-scatter grid 32, relative to the radiographic system via thefiducial markers 50 on the target array 28. An object to beradiographically imaged 1, such as a patient, is interspaced between thecollimator 24 and the grid tunnel 30. The image receptor 34 is placedproximal to anti-scatter grid 32 and distal from an object, such as apatient 1 as is known in the art (FIGS. 3 and 6A and 6B).

Other means may be employed to determine the position and orientation ofthe anti-scatter grid relative to the mobile radiographic unit. In suchalternate embodiment, the detector may be an ultrasound detector and thetarget array may comprise at least one ultrasound emitting element.Furthermore, the detector may be a magnetic dipole detector and thetarget array may comprise at least one magnetic element. In suchembodiment, the target array may be supported by a target arm or may beaffixed to the grid tunnel 30 or the anti-scatter grid 32. In yet afurther embodiment, a mechanical linkage arm equipped with rotationaland positional encoders, or combinations of accelerometers andgyroscopes mounted on the grid tunnel may be used. The use ofaccelerometers and gyroscopes, known as inertial navigation, measuresthe motion of the grid tunnel as it is moved from a dock fixedly mountedto the console 14 to its final position under the patient 1.

The following describes a specific embodiment of the grid tunnel 30, thetarget array 28 and the target arm 36. The following is exemplary innature only and is not meant to restrict the present disclosure to theembodiments shown and described. The grid tunnel 30 and the target arm36 are shown with greater clarity in FIGS. 5 and 6. The grid tunnel 30is manufactured from material selected from the group including, but notlimited to, rigid sheet metal, carbon fiber composites and impactresistant plastics, such as LEXAN (GE), polycarbonate, ABS and the like,or a combination of any of the above. In one embodiment, the grid tunnel30 is manufactured from carbon fiber composites. The grid tunnel 30 hassufficient strength to support the patient 1, and is typically designedto support more than 200 kilos. The grid tunnel 30 may have rounded edgesurfaces 38 to facilitate insertion under the patient 1. The arm 36supports the target array 28, and may be constructed of the samematerials as the grid tunnel 30. The arm 36 is adapted to insert withina channel 40 within the grid tunnel 30. A channel 40 may be providedalong adjacent edges of the grid tunnel 30 to accommodate transverse(parallel to the short axis of the grid tunnel 30) and longitudinal(parallel to the long axis of the grid tunnel 30) orientations of thegrid tunnel under the patient 1. Therefore, the target array 28 isplaced in a fixed position and orientation relative to the anti-scattergrid 32; however, as it apparent from the foregoing, the target arraymay be removed from the grid tunnel 30 and may be positioned at one ofseveral fixed orientations relative to the anti-scatter grid 32. Theterm fixed position and orientation should not imply the target array 28is permanently coupled to the grid tunnel 30 or is permanently in onlyone fixed orientation with regard to the anti-scatter grid 32.Optionally, a hand grip may be included in the grid tunnel 30 tofacilitate crude alignment of the grid tunnel 30 beneath patient 1. Inone embodiment, the target array 28 extends a distance from the gridtunnel 30 to ensure visibility when a large patient 1 covers the gridtunnel 30.

FIG. 5 shows one embodiment of a target array 28 according to thepresent disclosure having a plurality of fiducial markers 50, theposition of the fiducial markers 50 being fixed relative to theanti-scatter grid 32. In the embodiment illustrated in FIG. 5, threefiducial markers 52 are provided in a plane having a known position andorientation relative to the plane of the anti-scatter grid 32 to providea measure of the distance from the target array 28 to the detector(illustrated as camera 26), and therefore, the x-ray tube housing 22. Afourth fiducial marker 53 is placed out of plane relative to the markers52 provides a measure of transverse misalignment. The fiducial markers50 may comprise a variety of materials. In one embodiment, the fiducialmarkers 50 are a light reflecting substance. In a specific embodiment,the light reflecting substance is a retro-reflecting material thattransmits the reflected light in the direction in which the lightimpacted the material. In an alternate embodiment, the light reflectingsubstance is a corner mirror. The detector acquires an image of thetarget array 28 with a light source shining on the target array 28 (thelight source 25 may be mounted on the detector 26 or other components ofthe mobile radiographic system 10) and acquires a second image with nolight source shining on the target array 28. The processor subtracts thesecond image from the first image to produce an image that consistsessentially of the light reflected back by the light reflecting materialcomprising the fiducial markers 50. This process increases the contrastbetween the fiducial markers and the background substantially. In analternate embodiment, LEDs may be used as the fiducial markers. The useof LEDs as the fiducial marker is described in U.S. Pat. No. 6,702,459.The use of the light reflecting materials allows the use of a passivetarget arm 36 (without internal electronics and the ability tocommunicate with the processor) and greatly simplifies the constructionof the target arm 36 and avoids any requirement of communication betweenthe target array 28, the target arm 36 and the processor.

The processor analyzes images of the target array 28 acquired by thedetector to determine the position and orientation of the target array28 (which is equivalent to the position and orientation of theanti-scatter grid 32 in the grid tunnel 30) relative to a position onthe mobile radiographic system 10. The processor then calculates theoptimal position and orientation of the x-ray tube housing 22 such thatthe focal spot and/or central ray are in a state of alignment withregard to the anti-scatter grid 32 (FIG. 4). The motion control systemdirects the components of the mobile radiographic system 10 into a stateof alignment.

The motion control system is manually operated by the operator with theaid of the processor and the directing element, which may alert theoperator when at least one of the degrees of freedom of the mobileradiographic system is in the proper position. For example, thedirecting element may be a visual display which alerts the operator whenthe components are in the proper position. The processor may also lockany one of the degrees of freedom when it has the desired value. Inanother alternate embodiment, the motion control system is partiallymanually operated with the aid of the processor and directing elementand is partially manually operated without the aid of the processor anddirecting element. In yet another alternate embodiment, one or more ofthe degrees of freedom is manually operated with the aid of theprocessor and directing element, and one or more of the degrees offreedom is automatically operated by the processor and an automaticdrive system, such as a stepper or linear motor.

Referring to FIG. 4, the optimal location 42 of the focal spot is theintersection of the focal line 44 of the anti-scatter grid 32 and a line46 normal to the surface of the anti-scatter grid 32 that passes throughthe center of the anti-scatter grid 32. This location is defined as theoptimal focal spot position, and when the focal spot is in this locationthe transmission of x-rays through the anti-scatter grid 32 is at itsmaximum value. The x-ray source assembly is in its optimal orientationwhen the central ray of the x-ray beam passes through the center of theanti-scatter grid 32, and the long and short axes of the x-ray beam areparallel to the long and short axes of the grid tunnel 30. When thex-ray focal spot is in the optimal focal spot position 42 and the x-raysource assembly is in its optimal orientation, then the mobileradiographic system 10 is defined to have optimal alignment. The degreesof freedom of the mobile radiographic system may be moved appropriatelyto place the x-ray source assembly into an optimal alignment.

The focal spot is in an acceptable position if the transmission ofprimary x-rays through the grid is at least 90% of its maximum valueover the entire anti-scatter grid 32, and if the focal spot is within 5cm of the optimal focal spot position 42 in a direction parallel to thefocal line 44. For example, for a standard size 12:1 anti-scatter grid32 with a focal length of 100 centimeters, the focal spot position willbe acceptable if it is on the focal line 44 and within 5 centimetersfrom the optimal focal spot position 42, on the normal line 46 andwithin 2 centimeters from the optimal focal spot position 42, or on aline 45 normal to both grid focal line 44 and the grid normal line 46and within 0.8 centimeters of the optimal focal spot position 42.Similarly, the x-ray source assembly is in an acceptable orientation ifthe central ray of the collimated x-ray beam passes substantially closeto the center of the grid, and the long and short axes of the collimatedx-ray beam are substantially parallel to the long and short axes of thegrid tunnel 30. When the x-ray focal spot is an acceptable position, andthe x-ray source assembly is in an acceptable orientation, the system isdefined to have acceptable alignment. The degrees of freedom of themobile radiographic system may be moved appropriately to place the x-raysource assembly into an acceptable alignment.

While the detector 26 is illustrated affixed to the collimator housing24, it is appreciated that the detector according to the presentdisclosure can be mounted in a variety of positions on a mobileradiographic system 10. It is further recognized that other detectors inaddition to an optical detector, such as a camera, are operative herein.Such alternate detectors may be optical in nature, or be based on otherprinciples such as magnetic interactions or ultrasound. Some of thesedetectors mat not require the target array 28, but may directly detectthe grid tunnel 30 or target array 28, or fiducial markers attacheddirectly to the grid tunnel 30 or target array 28.

In operation according to the present disclosure, grid tunnel 30,containing the anti-scatter grid 28 and the image receptor 34, is placedunder an object to be radiographically imaged 1, such as a hospitalpatient. A radiological technician thereafter attaches the target arm 36to the grid tunnel 30. The arm 36 fits into a channel 40 on grid tunnel30 and extends past the lateral dimensions of the object 1. Thus, theend of the arm 36 is visible to the detector 26. The operator places thedetector 26 in rough alignment with the target array 28. The roughalignment process may be aided by the use of a positioning element onthe detector 26, such as the light source 25. After the rough alignment,a measuring protocol is activated by the operator. The detector 26collects an image of the target array 28 and its fiducial markers 50 andtransmits the data comprising the image to the processor. The processorcalculates the position and orientation of the target array 28 andassociated grid tunnel 30 using the fiducial markers 50, and thereforethe anti-scatter grid 32, relative to the detector 26. Once theprocessor calculates the relative position and orientation information,the operator activates the motion control system. On activation of themotion control system, the processor and the directing element thendirect the operator to move the system to a state of alignment asdetermined by the processor. The detector may collect a confirmatoryimage of the target array 28 to assure proper alignment of the mobileradiographic system.

The visual display 60 may alert the operator of the condition of thesystem 10 in addition to providing information regarding the currentand/or desired position of the components of the mobile radiographysystem. For instance, the visual display 60 could indicate the detector26 is unable to “see” all the fiducial markers 50 of the target array28, or the detector “sees” all the fiducial markers 50, but is not yetaligned or that the system is ready for use. Other information may alsobe displayed. This status information could also be displayed in otherways, for instance with a series of LED lights. In addition, the mobileradiographic system 10 may have at least one control element, such as abutton, toggle switch or similar device. The control elements may servevarious functions as desired. For example, one control element mayrelease the motion control system and allow the operator to roughlyalign the tube housing 22 with the target array 28. Another controlelement may activate the motion control system for alignment.

Before the system is used clinically, the mobile radiographic system 10undergoes calibration. This calibration step needs be performed justonce for a given mobile radiographic system 10 and target array 28, asthe calibration information is stored in a calibration file. The firststep in the calibration is to generate a correction for the spatialnon-linearity of the detector 26. This is accomplished by acquiring animage of a matrix of black dots, and fitting the measured position ofthe dots to a mathematical function. Next, the detector 26 is mounted onthe collimator 24, the target array 28 is mounted on the grid tunnel 30,and the tube housing 22 is positioned optimally so the x-ray focal spotfalls on the focal line of the anti-scatter grid 32 as described herein(an optimal state of alignment). The techniques involved in centeringthe focal spot are common the field and are within the ordinary skill ofone in the art. An image of the target array 28 is acquired and theprocessor then measures the position and orientation of the fiducialmarkers 50 on the target array 28 relative to the detector. The positionof the tube housing is then determined. The results of this measurementare stored in the processor. The process is depicted diagrammatically inFIG. 7.

The first step in the clinical alignment procedure is to determine theposition and orientation of the anti-scatter grid 32 relative to thetube housing 22 through the measurement of the position and orientationof the fiducial markers 50 on the target array 28. The detector 26acquires an image of the target array 28 and transmits the data to theprocessor as described herein. The processor also receives data from thepositional encoders in the components of the mobile radiographic system,which allows the processor to know the position of said components. Theprocessor takes this data and calculates the position of the tubehousing 22 relative to the console 14 so that the tube housing 22 willbe in a state of alignment relative to the grid tunnel 30 and theanti-scatter grid 32, that is, the relative position stored during thecalibration of the tube housing 22 (described above). The processor thenprovides this information to the display. The display then directs theoperator regarding the movement of the tube housing 22 to the positiondetermined by the processor so that the focal spot is in a state ofalignment with the anti-scatter grid 32. In summary, the processordetermines the position and orientation of the fiducial markers 50 ofthe target array 28 in relation to the x-ray tube housing 22, and usesthis information to calculate a state of alignment for the mobileradiographic system 10, and the display, directed by the processor,directs an operator in the movement of the component of the mobileradiographic system to a state of alignment.

The x-ray source assembly has six degrees of freedom. Three degrees offreedom allow the x-ray source assembly to move to the central positionon the focal line of the anti-scatter grid 32 (R, H, and Φ), two degreesof freedom allow the x-ray source assembly to direct the central ray ofthe x-ray beam to the center of the anti-scatter grid 32 (Θ and Ψ) andone degree of freedom allows the collimator 24 to align with the longaxis of the image receptor (Ω) (discussed in more detail below). Theprocessor measures the six Cartesian coordinates of the target array 28in relation to the detector 26. Encoders in the mobile radiographicsystem measure the values of the six degrees of freedom and convey thisinformation to the processor, which thereby determines the Cartesiancoordinates of the detector 26 relative to the console 14. The processorcompares the two sets of Cartesian coordinates and determines the sixCartesian coordinates of the target array 28 in relation to the console14. The processor then determines the six Cartesian coordinates of thedetector when the tube housing 22 is optimally aligned with theanti-scatter grid. Finally, the processor determines the values of thesix degrees of freedom when the tube housing 22 is in a state ofalignment.

It is appreciated that an acceptable degree of alignment can beaccomplished with fewer than six degrees of freedom. For example, withthree degrees of freedom (R, H, and Φ), the focal spot could be placedat the center point on the focal line of the grid, aligning the focalspot with the grid. The operator could then manually adjust theremaining degrees of freedom without the aid of the processor and visualdisplay. Generally, the collimator orientation (Ω) and the sourceassembly rotation adjustments (Θ and Ψ) are generally small and lesscritical and can be done without the aid of the processor and display ifdesired. In principle, the focal spot could be moved onto the focal linewith as few as two degrees of freedom, although with no guarantee thatit would fall close to the line normal to the center of the grid. Suchapproaches align the source assembly and grid at the expense of moreeffort on the part of the user. Therefore, while the foregoing hasdescribed all six degrees of freedom being associated with a positionalencoder in communication with the processor, the foregoing does notrequire that each of the six degrees of freedom have an associatedpositional encoder. For example, in one embodiment only three degrees offreedom may have an associated positional encoder, such as for example,R, H, and Φ. In this embodiment, only those components associated withthe three degrees of freedom would be displayed by the processor on thedisplay.

FIG. 9 diagrammatically illustrates one embodiment of the steps involvedin measuring the position of the tube housing 22. FIG. 10 shows anexample of the image acquisition process. An image is acquired from thedetector 26. The process may be varied depending on the nature of thefiducial markers 50. When the fiducial markers are light reflectivesubstances, the detector acquires a first image (referred to asforeground) of the target array 28 with a light source 25 shining on thetarget array 28 (the light source may be mounted on the detector 26 orother components of the mobile radiographic system 10) and acquires asecond image (referred to as background) with no light source shining onthe target array 28. The processor subtracts the background image fromthe foreground image to produce an image that consists entirely of thelight reflected back by the light reflecting material comprising thefiducial markers 50. This process increases the contrast of the fiducialmarkers relative to the background substantially. Alternately, only asingle image may be acquired with the light source turned on and thefiducial markers located by the increase in intensity created by thelight reflecting material. However, it is possible for any brightobjects in the background to confuse the processor in this technique.

The images acquired show the fiducial markers as areas of greaterintensity as compared to the background. The acquired image iscompressed in order to more efficiently locate the fiducial markers. Thefiducial markers are then located in the compressed image, and theneighborhood (i.e., general area) of the fiducial markers is identified.This neighborhood is scanned in the uncompressed image to identify theexact position of the fiducial markers. One embodiment of a sequence forlocating fiducial markers is shown in FIG. 11. The images are gammacorrected so that the pixel values are proportional to the intensity ofthe fiducial markers. The gains of the first and second images are thenmatched. The background image is then subtracted from the foregroundimage. A threshold is then applied to the difference image and pixelswith intensities above the thresholds are marked as possible candidatesfor the location of a fiducial marker. The fiducial marker candidatesare traced and analyzed, and candidates that do not meet certaincriteria (for example, size, shape, color, intensity, etc.) arediscarded. Finally, a list of candidate fiducial marker positions isreturned to the calling process.

By locating the general position of the fiducial markers in a compressedimage, the speed of the process is greatly increased. Any fiducialmarkers in the neighborhood are identified and added to a final list offiducial marker locations. The identification steps are repeated untilall fiducial markers are located and added to the final list of fiducialmarkers locations. If the final list does not contain exactly therequired number of fiducial markers, the measurement process isterminated and an error light may be displayed. If the correct number offiducial markers is in the final list, the tube housing 22 position iscalculated (as shown in FIGS. 9 and 11).

FIGS. 9 and 12 describe how the fiducial marker information is analyzedto determine the position of the detector 26 relative to the targetarray 28, and by inference the position and orientation of the tubehousing 22 relative to the anti-scatter grid 32. First, the detectorlinearity calibration is used to convert the centroid of each fiducialmarker (in pixel units) to a physical position (in cm) projected onto afiducial image plane. The Marquardt algorithm is used to calculate theposition of the target array relative to the tube housing 22 from thesemeasurements. The Marquardt algorithm is a general iterative algorithmfor fitting a non-linear function to a set of data, starting from aninitial estimate of the function parameters. The implementationgenerates the initial estimate by assuming that the distance to thedetector is infinite, and that the magnification of the detector imageis unknown. The iterations continue until a convergence criterion isreached. The final estimated parameters are considered good if themeasured and estimated fiducial marker positions match to within somelimit (e.g. 0.05 cm). The mathematics involved in the calculation of thealgorithm to convert the position measurements of the fiducial markersto a desired position and orientation of the tube housing 22 involveCartesian coordinate transformation. The details of this field ofmathematics are well known to those of ordinary skill in the art. Asnoted above, the processor uses this information to determine thedesired values of the various degrees of freedom of the system.

In response to calculation of the position and orientation information,the processor provides this information to the directing element, suchas a visual display, which directs an operator to adjust at least onedegree of freedom to move the x-ray tube housing to a state of alignmentwith the anti-scatter grid.

In one embodiment the x-ray tube housing 22 has six degrees of freedom,as discussed above. Three degrees of freedom correspond to the threespatial dimensions of the focal spot location (R, H, and Φ), two degreesof freedom correspond to the direction (altitude and azimuth) of thecentral ray of the X-ray beam (Θ and Ψ), and one degree of freedomcorresponds to a rotation of the collimator around the X-ray beam (Ω). Aportion of the motion control system as well as a positional encoder maybe associated with each degree of freedom to direct this motion underthe direction of the processor. However, as discussed above, fewer thanall six degrees of freedom may be associated with a positional encoder.

In one embodiment (FIG. 13), the X-ray tube housing 22 is mounted in agimbal 23. The gimbal 23 is mounted on a horizontal extensible arm 20,which in turn is mounted to a vertical column 16. The X-ray collimatorhousing 24 is movably mounted on the x-ray tube housing 22. Each ofthese degrees of freedom will be described briefly and are illustratedin FIG. 13. The arm 20 can be extended or retracted (motion R), thecolumn 16 can be moved up and down (motion H), and the column can berotated about a vertical axis (motion Φ). The three motions R, H, and Φtogether provide the three degrees of freedom necessary to locate thecenter of the x-ray tube housing 22 on gimbal 23 at a desired spatiallocation.

Once the x-ray tube housing 22 on gimbal 23 is located at a desiredlocation, the two bearings of the gimbal can be rotated (motions Θ andΨ) defining two additional directional degrees of freedom. If the focalspot is located at the intersection of the axes of motions Θ and Ψ, thenits position is determined uniquely by motions H, R, and Φ. Otherwise,the position of the focal spot is determined also by motions Θ and Ψ aswell. The last degree of freedom lies in the rotation Ω of thecollimator housing 24 around the central ray of the X-ray beam.

The processor directs the operator regarding the direction and amount ofmovement of at least one freedom of movement H, Φ, R, Θ, Ψ and Ω, toplace the x-ray tube housing in a state of alignment. The information isprovided to the operator by the directing element, such as visualdisplay 60. These movements could be accomplished by the operatorsequentially or in parallel. Parallel movement might have the advantageof reduced alignment time, but the disadvantage of increased complexityand possible distraction of the technologist by a relatively complexmotion. In one embodiment, the processor provides the operator with thedirection and amount of movement in the degree of freedom H, Φ and R toplace the gimbal in its desired location. Once the gimbal is in itsdesired location, the processor provides the operator with the directionand amount of movement for the three remaining freedoms of motion, Θ, Ψand Ω, to orient the x-ray beam and collimator to a state of alignment.As stated above, the processor need not provide direction and amount ofmovement information for each degree of freedom incorporated in themobile radiographic system.

As discussed herein, the movement directed by the processor is manualmovement, requiring the operator to physically move one or more degreesof freedom of the mobile radiography system.

In one embodiment, drive elements are associated with one or more of thedegrees of freedom are manual drive elements, such as but not limited togears, pulleys or other elements known in the art for accomplishingmovement. It should be noted that the processor is not required tocommunicate information to the drive elements of the motion controlsystem; the processor may simply provide, via the directing element, thedirection and amount of movement required for a degree of freedom so asto direct the operator in the manual movement. In the embodimentillustrated, the directing element is a visual display 60 incommunication with the processor. The display 60 provides the directionand amount of movement in one or more of said degrees of freedom toposition the components of the mobile radiography system to theirdesired location (which may be a state of alignment). The operator usesthis information to manually move the x-ray source assembly according tothe information on the display. In one embodiment when the operator usessequential movement, the display may show the required change in onedegree of freedom in order to move the selected degree of freedom to adesired position. The operator manually adjusts this degree of freedom,with the display continuously updating the required movement, until thedegree of freedom is in the desired location. At this point, theprocessor may optionally direct the motion control system to lock thedegree of freedom in place or the operator may manually lock the degreeof freedom in place. The display then shows the required change in thenext degree of freedom in order to move the selected degree of freedomto a desired position. Again, operator manually adjusts this degree offreedom, with the display continuously updating the required movement,until the degree of freedom is in the desired location. The processormay again lock the degree of freedom in place or the operator maymanually lock the degree of freedom in place. The process is repeatedfor the remaining degrees of freedom until all the degrees of freedom ofthe mobile radiography system are in the desired location. In oneembodiment, the display shows the desired movement for at least onedegree of freedom. In an alternate embodiment, the display shows thedesired movement for two or more degrees of freedom. In yet anotherembodiment, the display shows the desired movement for all degrees offreedom.

In an alternate embodiment when the operator uses parallel movement, thedisplay may show the required change in more than one degree of freedomin order to move the selected freedoms of motion to a desired position.The operator manually adjusts these degrees of freedom, with the displaycontinuously updating the required movement, until the freedoms ofmotion are in the desired location. When one or more freedoms of motionare in the desired location, the processor may optionally direct themotion control system to lock theses freedoms of motion in place or theoperator may manually lock these freedoms of motion in place. Any degreeof freedom that is not yet in a desired position will remain unlockeduntil the desired position is achieved.

In yet another embodiment, the operator may manually move at least onedegree of freedom as directed by the directing element under the controlof the processor as described herein and at least one degree of freedommay be automatically moved by the motion control system as described inU.S. Pat. No. 6,702,459. In this embodiment, a manual motion controlsystem and an automatic motion control system are present in the mobileradiography system. The automatic motion control system comprising anautomatic drive element, such as but not limited to a servo motor, forat least one of said degree of freedom in communication with saidprocessor. The processor thereby directs the movement of said at leastone degree of freedom without operator involvement.

Optionally, the operating console 14 is equipped with an inner lockdisabling the x-ray exposure until the x-ray focal spot and grid havebeen aligned according to the present disclosure. Further, it isappreciated that an increase in tube voltage is expected to provideimproved images as compared to imaging done absent an anti-scatter grid.The increase in tube voltage is intended to increase x-ray transmissionthrough the patient 1 and thereby allow a shorter exposure time.Optionally, a mobile radiographic system according to the presentdisclosure is provided with an alarm system which is activated uponmovement of the mobile radiographic system 10 absent grid tunnel 30 toprevent accidental loss of the grid tunnel 30 and the target arm 28.

It is appreciated that localization techniques can be performed not onlyby the optical methods detailed herein, but also through the use ofmagnetic dipole technology and/or ultrasound technology. Magnetic dipolearrays and sensors operating with the benefit of current loops orelectromagnets are detailed in U.S. Pat. No. 4,054,881.

Patents and publications mentioned in this specification are indicativeof the levels of those skilled in the art to which the disclosurepertains. These patents and publications are incorporated herein byreference to the same extent as if each individual patent or publicationwas specifically and individually incorporated herein by reference.

The foregoing description is illustrative of particular embodiments ofthe invention, but is not meant to be a limitation upon the practicethereof. The following claims, including all equivalents thereof, areintended to define the scope of the invention.

REFERENCES

-   1. Barnes G T: Contrast and Scatter in X-Ray Imaging. RadioGraphics    11:307-323, 1991.-   2. Niklason L T, Sorenson J A, Nelson J A: Scattered Radiation in    Chest Radiography. Med. Phys. 8:677-681, 1981.-   3. Tucker D M, Souto M, Barnes G T: Scatter in Computed Radiology.    Radiology 188:271-274, 1993.-   4. Niklason L T, Barnes G T, Carson P: Accurate Alignment Device for    Portable Radiography. Radiology 173(P):452, 1989.-   5. O'Donovan P B, Skipper G J, Litchney J C, Salupo A J, Bortnick J    R: Device for Facilitating Precise Alignment in Bedside Radiography.    Radiology 184:284-285, 1992.-   6. Press W H, Flannery B P, Teukolsky S A, Vetterling W T, Numerical    Recipes in C: The Art of Scientific Computing, Cambridge University    Press, Cambridge UK, 1988.

1. A mobile radiographic system, the system comprising: a. an x-raysource assembly, said x-ray source assembly comprising an x-ray tubehousing having an x-ray source with a focal spot, a tube housingmounting adjustably supporting said x-ray tube housing, said x-ray tubehousing adjustably mounted to an adjustable support and an x-raycollimator adjustably coupled to the x-ray tube housing, said x-raysource assembly being adjustable with regard to at least one degree offreedom allowing the x-ray tube housing to be moved to a desiredlocation and orientation with respect to an anti-scatter grid; b. saidanti-scatter grid not being in a fixed position and orientation relativeto the x-ray tube housing; c. a measuring system, said measuring systemcomprising a positional encoder for at least one of said degrees offreedom and a detector for acquiring position and orientationinformation of the anti-scatter grid relative to a fixed point on themobile radiographic system; d. a manual motion control system to directthe movement of at least three of said degrees of freedom, said motioncontrol system comprising a drive element for at least one of saiddegrees of freedom and a directing element; e. a processor incommunication with said detector, said encoder and said directingelement, the processor determining the desired value of the at least onedegree of freedom so that the x-ray tube housing will be in a state ofalignment, the processor not being in communication with the driveelement, wherein an operator sequentially adjusts groups of degrees offreedom, adjusting in parallel the degrees of freedom within each group,where each degree of freedom is manually adjusted by the operator asdirected by the directing element.
 2. The system of claim 1 where eachsaid degree of freedom is locked in place by the processor or theoperator once said desired value is achieved.
 3. The system of claim 1where at least one of the group of degrees of freedom comprise a singledegree of freedom, and at least one of the degrees of freedom is notlocked in place by the processor or the operator once said desired valueis achieved.
 4. The system of claim 1 where the directing elementprovides the current location, the desired location of said at least onedegree of freedom.
 5. The system of claim 1 where said directing elementprovides a direction and an amount of movement of said at least onedegree of freedom to adjust said at least one degree of freedom to saiddesired value.
 6. The method of claim 1 where the directing element isvisual in nature, tactile in nature or auditory in nature.
 7. The methodof claim 6 where the directing element is a visual display.
 8. Thesystem of claim 1 where said at least three degrees of freedom are H, Rand Φ.
 9. The system of claim 8 where said at least three degrees offreedom are locked in place by the processor or the operator once saiddesired values are achieved.
 10. The system of claim 1 where saidmeasuring system comprises a positional encoder for each of said six ofsaid degrees of freedom.
 11. The system of claim 10 where an operatormoves said six degrees of freedom as instructed by the directing elementto adjust said six degrees of freedom to said desired values.
 12. Thesystem of claim 10 where said six degrees of freedom are locked in placeby the processor or the operator once said desired values are achieved.13. The system of claim 1 where the detecting element comprises a targetarray having a plurality of fiducial markers in a fixed positionrelative to the anti-scatter grid, and a detector acquiring position andorientation information of the target array relative to the mobileradiographic system.
 14. The system of claim 13 where the detector ismounted on the collimator or on the x-ray tube housing.
 15. The systemof claim 13 where the fiducial markers are light reflecting targets,magnetic targets, sound emitting targets, variously colored lightemitting diodes or monochrome light emitting diodes.
 16. The system ofclaim 13 where the fiducial markers are light reflecting targets and thesystem further comprises a light source to illuminate said lightreflecting targets.
 17. The system of claim 16 where said lightreflecting targets are manufactured from a retro-reflective material.18. The system of claim 13 where said fiducial markers are separated bya known distance.
 19. The system of claim 13 where said fiducial markersare not located in one plane.
 20. The system of claim 13 where thedetector is an optical detector, a magnetic detector or an ultrasounddetector.
 21. The system of claim 20 where the optical detector is astill frame digital camera, a digital video camera or an analog videocamera.
 22. The system of claim 1 where said desired location andorientation is a state of alignment.
 23. The system of claim 22 wheresaid state of alignment is an optimal state of alignment or anacceptable state of alignment.
 24. The system of claim 1 where theanti-scatter grid is contained within a grid tunnel.